Method and arrangement for producing a basic element of a multi-element optical cable

ABSTRACT

A method and arrangement for producing a basic element of a multi-element fibre optic cable having a coating tool (6) for forming a tubular sheathing around a single optical fibre (3) or a fibre bundle formed of several optical fibres (3), and a pulling device (8) placed after the coating tool (6) for pulling the sheathing and the fibres forward as a unit. In order to produce a stable product, the tension (t 3 ) of the fibres (3) passed to the coating tool (6) is kept continuously at a controlled level. The direction of travel of the fibres and the sheathing is kept straight before and at the pulling device (8), and the pulling device (8) is spaced from the coating tool (6) at a distance depending on the production rate of the production line, so that desired sliding of the fibres (30 inside the sheathing is provided at the pulling device (8).

FIELD OF THE INVENTION

The invention relates to a method for producing a basic element of amulti-element optical cable, in which method a tubular sheathing isextruded loosely around a fibre or a fibre bundle consisting of severaloptical fibres, the fibres are provided with the desired excess lengthwith respect to the sheathing, and the unit formed by the sheathing andthe fibres is pulled forward from the place following the point ofextrusion to the next stage in the production line. The invention alsorelates to an arrangement for producing a basic element of amulti-element optical cable.

BACKGROUND

Such methods and arrangements are rather well known at present inconnection with producing optical cables. Examples of known solutionsinclude arrangements described in U.S. Pat. No. 4,893,998, German patent3,425,649 and European patent application 0,286,819.

The disadvantages of the aforementioned arrangements include for examplethat the direction of travel of the unit formed by the fibres and thetubular sheathing changes rather rapidly after the extrusion head, sincethe unit is passed around a so-called locking wheel. In a productionline equipped with a locking wheel the amount of the fibres is limited,since otherwise the fibres are positioned one upon another on thelocking wheel, whereupon the outermost fibres may break at the worst. Inorder to operate reliably especially under conditions where thetemperature fluctuates, for example an SZ stranded cable having aso-called minipipe structure requires great stability of each componentof the cable and controlled interaction between the components. Examplesof matters that cannot be realized in the best possible manner with theprior arrangements include the levelling of the variation in the lengthof the fibres situated inside the same protective pipe or sheathing,which is the most important factor in an SZ stranded multi-fibrestructure, wherefore controlled levelling during the manufacture of thecable is important for the final result. The levelling of the variationin the fibre length is controlled by regulating the mutual tension ofthe fibres. Another example concerns the dimensions of the pipe orsheathing acting as a protective housing. A third example relates to therelaxation following the manufacture of an individual protective pipe orsheathing and the control of the relaxation during the manufacture. Thestability must be absolute in an SZ stranded structure.

SUMMARY OF THE INVENTION

The purpose of the invention is to provide a method and an arrangementwith which the drawbacks of the prior solutions can be eliminated. Thisis achieved with the method and arrangement according to the invention.The method according to the invention is characterized in that thetension of the fibres passed to the point of coating is maintainedcontinuously at a controlled level, that the direction of travel of thefibres and the sheathing is kept straight before the pulling point andat the pulling point, and that the pulling point is placed from thecoating point at a distance depending on the production rate of theproduction line. The arrangement according to the invention is in turncharacterized in that the tension of the fibres passed to the coatingtool is arranged to be kept continuously at a controlled level, that thedirection of travel of the fibres and the sheathing is arranged toremain straight before the pulling device and at the pulling device, andthat the pulling device is placed from the coating tool at a distancedepending on the production rate of the production line.

The primary advantage of the invention is that it enables an increase ofabout 200 to 300% in the production compared to the results achievedwith the prior arrangements. It must be noted that the aforementionedincrease in the production is achieved without sacrificing the existingcriteria for quality. The invention makes it possible to increase theproduction rate manyfold compared to the present rates of production bymeans of development and improved materials. Another advantage of theinvention is its simplicity, wherefore the introduction and use of theinvention will be advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below by means of apreferred embodiment shown in the accompanying drawing, in which theonly FIGURE is a schematic side view of the production line of aminipipe utilizing the invention. A minipipe refers to an element of astranded multi-element cable.

DETAILED DESCRIPTION

The drawing FIGURE shows schematically the production line of a minipipeutilizing the invention. Reference numeral 1 generally denotes fibresupplies with tension control devices contained therein. Referencenumeral 2 denotes guide wheels that are arranged to guide the fibres 3to a fibre tension control device 4. The fibre tension control device 4is as shown the FIGURE a device consisting of two successive wheels 4a,4b, described in greater detail in Finnish patent application 953,338filed simultaneously with the present application and corresponding U.S.application Ser. No. 08/981,579. It should be noted, however, that thepresent invention is not limited in any way to the tension controldevice shown in the FIGURE, but any other suitable apparatus may also beused as the tension control device. Reference numeral 5 denotes a deviceused for measuring the line tension of the fibres. The initial tensionof the fibres is denoted in the FIGURE with t₁, the tension between thewheels 4a, 4b is denoted by t₂, and the tension of the fibres in theline by t₃.

The fibres 3 are passed from the starting reels to a coating tool 6placed in the point of coating as shown in the FIGURE, the tool possiblycomprising an extrusion head of an extrusion apparatus. By means of thecoating tool 6, a loose tubular sheathing is extruded around the fibrebundle. At the point of coating, a filling gel is supplied inside thepipe acting as the sheathing by supply means 7 for filling gel. Thecoating tool 6, the supply means 7 for filling gel, and the compositionof the gel constitute technology that is fully conventional for a personskilled in the art, wherefore they will not be described in greaterdetail in this connection.

After the coating tool, the unit formed by the pipe and the fibres ispulled through a precooling device 9 by means of a pulling device 8.From the pulling device 8 the unit consisting of the pipe and the fibresis passed via a relaxation zone 11 and a final cooling zone 12 to areceiving spooler 13. Reference numeral 14 denotes a measuring wheel ofa pipe for ON-line excess length measurement of fibre (ON-line biasmeasurement), reference numeral 15 denotes a control dancer for the linetension of the pipe, and reference numeral 16 denotes a device,so-called line tension pillar, used for maintaining the line tension.Reference numeral 17 denotes a meter for measuring the diameter of thepipe.

The precooling temperature of the pipe is denoted in the FIGURE by T₁,the pipe relaxation temperature by T₂, and the final cooling temperatureof the pipe by T₃. The line tension of the pipe is denoted by t₄, andthe spooling tension of the pipe is in turn denoted by t₅. PT₁ denotes apulse tachometer used for measuring the fibre to be supplied inside, PT₂denotes a pulse tachometer for the speed point, and PT₃ denotes a pulsetachometer for measuring an outcoming pipe.

According to an essential idea of the invention, the tension of thefibres supplied to the point of coating, i.e. to the coating tool 6, ismaintained continuously at the desired level. With the controlledtension, it is possible to provide the adjustment of the excess lengthof the fibre. By means of the fibres and the coating tool, the directionof travel of the pipe or sheathing extruded around the fibres is keptstraight before the point of pulling. The pulling point is positionedfrom the coating point at a distance depending on the production rate ofthe production line, so that the desired sliding of the fibres 3 withrespect to the sheathing is provided at the point of pulling. Theposition of the pulling point can preferably be made adjustableaccording to the production rate of the production line. This can berealized for example in such a way that the pulling device 8 is placedin the line so that its position with respect to the coating tool 6 canbe changed. It can be mounted for example by means of suitable railstructures. A belt pulling device can be preferably used as the pullingdevice.

The operation of the invention is thus based on the controlled tensionof the fibre(s) and on the sliding of the fibre(s) inside the protectivepipe during the manufacture, which aims at providing a controlled excesslength for the fibre. The above-described matter can also be defined insuch a way that during the manufacture, the sliding of the fibre(s)compensates for the positiveness of the excess fibre caused by therelaxation and the thermal contraction of the pipe. It must be notedthat the continuous control of the fibre tension is the most importantcontrolled variable as regards both the sliding of the fibres and therecovery of tension. The sufficient controlled sliding of the fibres isonly possible in a straight line. In a line provided with a lockingwheel, the physical limits of the line restrict the sliding at too earlya stage.

The sliding of the fibres can be implemented when the pulling device 8in the line is a belt pulling device that is placed at a suitabledistance from the extrusion head 6 in view of the production rate of theline. With the controlled fibre tension, the amount of the fibre to besupplied can be adjusted so that the sufficient sliding of the fibreswith respect to the pipe can be implemented at the belt pulling device.

In the above-described manner, the positiveness caused by the thermalcontraction and the relaxation can be entirely compensated for incontrolling the fibre excess length of the pipe. The sliding of thefibres can be facilitated by means of the properties of the filling gelor by preheating the filling gel to a point where its viscositydecreases. The preheating is naturally performed within the limits setby the fibre. The final product is a pipe with no relaxation and withthe desired excess fibre on both sides of the zero point. The structuralbehaviour of the optic cable formed of elements prepared in theabove-described manner is also stable.

The above-described embodiment is not intended to restrict the inventionin any way, but the invention can be modified quite freely within thescope of the claims. Therefore it is clear that the arrangementaccording to the invention or the details thereof do not have to beexactly similar to those shown in the FIGURE, but other kinds ofarrangements are also possible.

What is claimed is:
 1. A method for producing a basic element of amulti-element optical cable, comprising extruding a tubular sheathing,at a point of extrusion loosely around an advancing optical fibercomponent, providing the fibre component with a desired excess lengthwith respect to the sheathing, pulling the sheathing and the fibrecomponent as a unit forward from pulling point located following thepoint of extrusion to a subsequent stage in a production line,continuously maintaining a tension (t₃) in the fibre component passed tothe point of extrusion at a controlled level, keeping the fibrecomponent and the sheathing in a straight direction of travel before andat the pulling point, and locating said pulling point at a distance fromthe point of extrusion at a distance depending on a production rate ofthe production line.
 2. A method according to claim 1, comprisingadjusting the position of the pulling point according to the productionrate of the production line.
 3. A method according to claim 1, whereinsaid optical fibre component is formed by one or more optical fibres. 4.An arrangement for producing a basic element of a multi-element opticalcable, said arrangement comprising a coating tool for forming a loosetubular sheathing around an optical fibre component advancing along aproduction line, means for providing the optical fibre component with adesired excess length with respect to the sheathing, and a pullingdevice placed after the coating tool for pulling the sheathing and theoptical fibre component as a unit forward to a subsequent stage in theproduction line, means for tensioning said optical fibre component, asthe fibre component passes to said coating tool, at a determinedconstant level, and means for keeping the direction of travel of thefibre component and the sheathing straight before and at the pullingdevice, said pulling device being placed at a distance from the coatingtool which is a function of a production rate of the production line. 5.The arrangement according to claim 4, wherein said pulling device ismovable in accordance with the production rate of the production line.6. The arrangement according to claim 4, wherein the pulling devicecomprises a belt pulling device.